Descaling process



Patented Sept. 25, 1934 1.9 4.511 UNITED STATES PATENT OFFICE.

1,974,571 DESCALING vrnoi'nlss George C, Kiefer, Springdale, Pa...assignor to Allegheny Steel Company, a corporation of Pennsylvania NoDrawing.

Application March as, 1932. Serial No. 601,889

- v 8 Claims. (Cl.148-8) the course of their manufacture. For thepurpose of this application, I will hereafter designate such articles,sheets, without, however, any intention of limiting the invention to theremoval of scale from sheets only.

The alloys with which this invention has particularly to do may begrouped within a classcontaining from 10% to chromium and from 25% to 5%nickel with the remainder principally iron, but which may contain smalladditions of vanadium; molybdenum, tungsten, copper, etc.-

The most widely 'used of the alloys within such roup is that alloy whichcontains in addition to iron, about 18% chromium and about 8% nickel.

The removal of scale acquired in rolling, heat treatments and otherworks operations is a very necessary and important part of themanufacture of sheets from such alloys and presents a highly complexproblem one which is much more dithcult of solution than is usuallyencountered in the descaling of other alloys met with inthe steelmakingart.

sheets from such alloys, the scale formed thereon has a tendency to jumpleaving a bare metal od of treating sheets formed from alloys of the"above group in such a manner as to completely remove the scale'or'oxidecoatings thereon without over-pickling anyportions of such sheets.

' While'the removal of scale from sheets: made from such alloys withoutover-pickling or etch-j ing some portions of the. same is very difficultwith the-pickling acids or solutions ordinarily employed for thispurpose such scale removal without overpickling or etching is madepossiblesolution as set forth in my copending application Serial No.424,143 filed January 28, 1930, for a Method of treating chromium-nickelalloy steels which wlll'not over-etch the sheets, 'no matter how longthe same are left in will entirely remove the scale or coating formedduring the heat treatment provided the heat treatment is carried out ina certain manner. 7 g g I have discovered that the heat treatment andthe pickling of articles made from alloys within the above group gohand-in-hand and that the type of heat treatment or manner of carryingout the same is of utmost importance in the results to be obtained bysubsequent pickling in my pickling solution or solutions, which maycon-v sist of any suitable or preferred composition such as a solutionpreferably heated, andcontaining about 25% commercial nitric acid andabout 2% hydrofluoric acid (48%) by volume.

While the atmosphere of the furnace withinwhich the heat treatment iscarried out has an efiect upon the results of the subsequent pickling, Ifind that this is not so important as the manner in which the sheets areheated, that is, the manner in which the sheets are brought to an- 4 ihealing temperature. During-certain stages in themanufacture of I findthat satisfactory results can be obtained in a furnace having a reducingatmosphere or in a mufilefurnace, the atmosphere of which 0on tains butlittle oxygen. ,7

I find that in order to obtain satisfactory results in pickling, it isnecessary to, so to speak, plunge the sheets intoa zone raised to theproper annealing temperature. By this I mean that it is necessary tomove them into such zone rapidly and that it is necessary to have thevolumetric capacity of such zone such that the sheets, as theyenter thezone, are immediately raised to the anwhere continuousconveyors areemployed.

by the proper solution or solutions provided such sheets are first heattreated in .a. certain manner.

In order to render such sheets relatively soft and easy to work, it isnecessary toraise the same toannealing temperature and then to quicklycool the same. This heating and cooling which I term a heat treatment asnow carried on, produces a scale or coating whichis only removed withgreat. difliculty with the ordinary acids or solutions now commonlyused. I

I have-discovered that a pickling solution either used as a single stepsolution or as a two step I find that it is necessary to heat treat thesheets, singly in order that they may become uniformly heated on bothsides.

I find that the niost satisfactory manner of so heating the sheets is toconvey them rapidly into the annealing'zone and there deposit them onsupport members raised to and maintained at annealing. temperature.

' I find that it is only necessary to keep the sheets within this zonefor a short time. I find that a mullie furnace having a door at one endonly or doors at opposite ends through which the sheets may be enteredand removed serves the purpose admirably. As soon as the sheets areheated to the proper temperature, they'can be removed to the air andthus cooled. Such cooling I term quick orrapid cooling, but if morerapid cooling is deemed necessary, the sheets can be either plunged intoa bath ormoved into a cold zone.

The scale formed during hot rolling is the scale which jumps, leavingthe sheets in a mottled condition, that is, with areas from which thescale has jumped and areas having a tightly adhering scale, and thistightly adhering scale must be heat treated before it can be removed bymy pickling solution or solutions.

The sheets after the last hot rolling operation are quickly heated toannealing temperature and then quickly cooled after which they aresubjected to my pickling solution or solutions.

After the scale has been removed and the sheets scrubbed withplain'water, they are cold rolled to approximate gauge and then annealedand pickled. This annealing is of the same type, that is, the sheets areplunged into the annealing zone, quickly cooled and then immersed in abath or baths of my pickling solution or solutions; thenthey are readyfor final finishing operations such as polishing, or grinding andpolishing. The annealing of the cold rolled sheets is preferably carriedon in a muiiie furnace.

The annealing of the hot rolled sheets is preferably carried on in anormalizing furnace in which a reducing atmosphere is maintained and inwhich the sheets are quickly heated to an annealing temperature andthereafter quickly cooled.

If it were not necessary to heat treat such alloy sheets after hotrolling, the scale on such sheets might be removed by my picklingsolution because the heating of the sheets in the sheet furnace for thefinal stage of hot rolling places the scale in condition tobe readilyremoved by such pickling solution.

After hot rolling, the sheets must be heat treated, and pickled and ifthereafter cold rolled they must again be heat treated and pickled afterthe cold rolling before they are suitable for commercial purposes.

The temperature to which I preferably raise the sheets during heattreatment is between 1850 and 1950 F. after which the sheets must bequickly cooled in order to render them ductile.

This treatment, in the case of hot rolled sheets from which-a portion ofthe scale has jumped, places such sheets in condition to have completescale removal by means of my pickling solution, and, in the case of coldrolled sheets, the heat treatment or heat treatments plus the coolingstep forms a scale which is readily removed by my pickling solutionwithout etching the surface of the sheets, no matter how long the sameare left in the solution. It.,is impossible to overpickle the sheets,but unless the sheets have been quickly heated to annealing temperaturein a reducing or non-oxidizing atmosphere, the

scale thereon cannot be readily removed by means of my picklingsolution. The heat treatment, therefore, in my opinion, forms animportant and integral part of my scale removing process.

The above is intended more in an illustrative than in a limitativemanner and I do not limit myself thereto except by the subjoined claims.

- My present application is in part a continuation of "my copendingapplication Serial No. 424,143, filed January 28, 1930 for a Method oftreating chromium-nickel alloy steels.

'iron alloy sheets or the like comprising rapidly heating such sheets toannealing temperature,

annealing them, rapidly cooling them and. then subjecting them to apickling bath consisting only of nitric and hydrofluoric acids.

2. A process for descaling chromium nickel iron alloy sheets or. thelike comprising rapidly heating such sheets to annealing temperature,annealing them, rapidly cooling them and then subjecting them to apickling bath consisting of about 25% commercial nitric acid and about2% hydrofluoric acid (48%) 3. A process for descaling chromium nickeliron alloy sheets or the like comprising rapidly heating such sheets toannealing temperature in an atmosphere of sub-normal oxygen content ascompared with the oxygen content in the air, annealing them, rapidlycooling them and then subjecting them to a pickling bath consisting ofabout 25% commercial nitric acid and about 2% hydrofluoric acid (48%) 4.That step in the process of descaling chromium nickel iron alloy sheetsand other chromium nickel iron alloy articles comprising subjecting thesame to a heat treatment which includes rapidly heating the same to1850-1950 F. in an atmosphere. containing less oxygen'than is containedin air and then rapidly cooling the same preliminary to subjecting suchsheets and articles to a pickling bath containing nitric andhydrofluoric acids but in which the nitric acid is the predominatingacid and thereafter pickling such sheets and articles in such bath.

5. That step in the process of descaling chromium nickel iron alloysheets and other chromium nickel iron alloy articles comprisingsubjecting the same to a heat treatment which includes rapidly heatingthe same to 1850-1950" F. in an atmosphere containing less oxygen thanis contained in air and then rapidly cooling the same preliminary tosubjecting such sheets and articles to a pickling bath containing nitricand hydrofluoric acids but in which the nitric acid is the predominatingacid and thereafter pickling such sheets and articles in such bath, saidchromium nickel iron alloy containing about 18% chromium and about 8%nickel.

6. In descaling chromium nickel iron alloy products, the new combinationof steps comprising rapidly annealing such alloys in an atmosphererelatively poor in oxygen as compared to ordinary air at 1850-4950 F.and then pickling in a nitric acid-hydrofluoric acid bath in whichnitric acid is the predominating acid.

'7. The method of treating sheet-like material made from an ironchromium nickel alloy having a chromium content of from about 10% toabout 30% and a nickel content of from about 25% toabout 5%,'comprisingrapidly raising the temperature of the material to from about 1850 -toabout 1950 F., then rapidly cooling the material, then subjecting thematerial to a pickling procedure in a bath containing about 25%commercial nitric acid and about 2% hydrofluoric acid (48%).

8. The method of treating sheet-like material idly'cooling the materialand then subjecting the material to a pickling procedure in a bathcontaining about 25% commercial nitric acid and about2% hydrofluoricacid .8%).

GEORGE C. KIEF'ER.

